It is known in gaseous fuelled, Diesel-cycle internal combustion engines to employ a pilot fuel to initiate combustion. Typical gaseous fuels, such as natural gas, have a low cetane number making auto-ignition in cylinders employing conventional compression ratios difficult. A pilot fuel, such as diesel, is introduced late in the compression stroke where it auto-ignites due to the pressure and temperature in the cylinder, thereby creating an environment suitable for the ignition of the gaseous fuel. In general, a pilot fuel can be any fuel that reliably auto-ignites with the desired timing in the conditions produced inside the combustion chamber during normal operation. For example, in an engine designed to be fuelled with diesel fuel, diesel is a suitable pilot fuel because it will auto-ignite at the temperatures and pressures produced inside the combustion chamber during the normal engine cycle.
It is known to introduce gaseous fuel in a Diesel-cycle internal combustion engine in multiple stages. The Applicant's own U.S. Pat. No. 6,640,773, issued Nov. 4, 2003 to Ancimer et al., discloses a homogenous charge direct injection engine that introduces a gaseous fuel and a pilot fuel in three stages. In a first stage, gaseous fuel is directly injected in the compression stroke such that it mixes with air forming a homogenous charge. In a second stage, a pilot fuel is introduced later in the compression stroke to initiate combustion of the homogenous charge. The amount of gaseous fuel introduced in the first stage is limited to reduce, and preferably prevent, the likelihood of premature combustion, such as engine knocking. For some parts of the engine's load and speed range not enough gaseous fuel is introduced in the first stage to meet the load requirements of the engine, and in such operating conditions gaseous fuel is directly injected in a third stage after the pilot fuel such that the load requirements of the engine are met.
The above technique has several advantages. There is a reduced likelihood of engine knock. The nitrous oxides (NOx) formation rate from combustion of the first stage gaseous fuel is very small since the premixed gaseous fuel and air burn under very lean conditions. It provides efficient operation by using stratified charge (pilot plume), lean burn and diffusion modes of combustion all in the same engine cycle, and maintains high engine output while reducing NOx emissions compared to conventional methods of operation. Nevertheless, in some implementations there are drawbacks with this technique, such as combustion stability, limited engine power and difficulties with engine calibration. Unburned hydrocarbon emissions may increase due to excessive leaning of the premixed charge.
In gaseous fuelled Diesel-cycle internal combustion engines that burn gaseous fuel in diffusion mode only, it is known that NOx formation rates can be excessive in certain regions of the load and speed range of the engine due to combustion temperature and pressure. For example, referring to FIG. 1 characteristic region 10 is illustrated for one internal combustion engine implementation where NOx formation rates are excessively high. The state of the art is lacking in techniques for reducing NOx formation rates in gaseous fuelled internal combustion engines that operate in a diffusion combustion mode. The present method and apparatus provide a technique for improving the operation of gaseous fuelled Diesel-cycle internal combustion engines operating in a diffusion combustion mode.